eMTC (or LTE Cat-M1), is a particular version of LTE (4G) wireless communications standard supported by 3GPP (and by major operators and equipment vendors within 3GPP). It aims to provide low-cost yet reliable connectivity for internet-of-things (IoT) and device-to-device communication applications. Its main characteristics include significantly increased battery life, coverage extension (i.e., connectivity even in very bad signal conditions), and low cost.
For eMTC devices operating in system bandwidths higher than 10 resource blocks (RBs), known architectures use either wideband (WB) zero-IF (direct conversion) receivers or narrowband (NB) zero-IF receivers.
In the WB approach, the radio local oscillator (LO) is tuned to the centre of the system bandwidth and all resource blocks of the system bandwidth are preserved and converted into the digital domain.
In the NB approach, which is cost-efficient and power-efficient compared to WB architectures, the receiver exploits the fact that transmission is done in ‘narrowbands’ (NB) within a given system bandwidth. eMTC uses narrowbands of 72 subcarriers (6 resource blocks) with subcarrier spacing 15 kHz. The local oscillator is tuned to the centre of the 6 RB sub-band. For example, a system bandwidth of 10 MHz (50 resource blocks) will contain 8 non-overlapping narrowbands of 1.08 MHz of 6 resource blocks each. A system bandwidth of 20 MHz (100 resource blocks) is divided into 16 non-overlapping narrowbands of 6 resource blocks each. It is noted that as 50 or 100 does not divide by 6, a subset of resource blocks remain unused, according to standard.
However, the consequence of using NB RF with direct conversion (zero-IF) is that the DC carrier needs to be punctured, i.e., all data transmitted on this carrier is considered as irrevocably lost due to being completely overshadowed by DC, and this results in significant performance degradation.
Typically, an eMTC device comprises Rx (receiver) and Tx (transmitter) chains, which have analog (RF), mixed-signal processing (such as ADC, DAC) as well as digital circuitry known as baseband signal processing, and software running on a dedicated digital signal processor, DSP.